Chemical mechanical polishing (CMP) has emerged as a crucial semiconductor technology, particularly for devices with critical dimensions smaller than 0.3 microns. One important aspect of CMP control is endpoint detection (EPD), i.e., determining when to terminate the polishing during the polishing process. The EPD systems are, in principle, in-situ EPD systems, which provide endpoint detection during the polishing process.
One class of prior art in-situ EPD techniques involve the electrical measurement of changes in the capacitance, the impedance, or the conductance of the test structure on the wafer and calculating the end point based on an analysis of this data.
Another electrical approach which has proven production worthy is to sense changes in the friction between the wafer being polished and the polish pad. Sensing changes in the motor current does such measurements. This method is only reliable for EPD for metal CMP because of the dissimilar coefficient between the polish pad and the tungsten-titanium nitride-titanium film stack versus the polish pad and the oxide underneath the metal. However, with advanced interconnection conductors such as polysilicon, oxide, copper, and barrier metals, e.g. tantalum or tantalum nitride, have a coefficient of friction similar to the underlying oxide. This approach relies on detecting the Cu-tantalum nitride transition, then adding an overpolish time. Intrinsic process variations in the thickness and composition of the remaining interfacial layer mean that the final endpoint trigger time is less precise than is desirable.
Another method uses an acoustic approach. In the first acoustic approach, an acoustic transducer generates an acoustic signal which propagates through the surface layer(s) of the wafer being polished. Some reflection occurs at the interface between the layers, and a sensor positioned to detect the reflected signals can be used to determine the thickness of the topmost layer as it is polished. The second acoustic approach is to use an acoustical sensor to detect the acoustical signals generated during CMP. Such signals have spectral and amplitude content which evolves during the course of the polish cycle. However, to date there has been no commercially available in situ endpoint detection system using acoustic methods to determine endpoint.
Finally, optical EPD systems as exemplified by U.S. Pat. No. 5,433,651 to Lustig et al. sense changes in a reflected optical signal using a window in the platen of a rotating CMP tool. However, the window complicates the CMP process because it presents to the wafer an inhomogeneity in the polish pad. Such a region can also accumulate slurry and polish debris.
U.S. Pat. No. 5,413,941 discloses a method in which the wafer is lifted off of the pad a small amount, and a light beam is directed between the wafer and the slurry coated pad. The light beam is incident at a small angle so that multiple reflections occur. The irregular topography on the wafer causes scattering, but if sufficient polishing is done prior to raising the carrier, then the wafer surface will be essentially flat and there will be very little scattering due to the topography on the wafer. The difficulty with this approach is that one must interrupt the normal process cycle to make the measurement.
U.S. Pat. No. 5,643,046 describes the use of monitoring absorption of particular wavelengths in the infrared spectrum of a beam that passes through a wafer being polished. Changes in the absorption within narrow, well defined spectral windows correspond to changing thickness of specific types of films.
Each of these above methods have drawbacks. What is needed is a new method for endpoint detection that is capable of operation in the manufacturing environment.